All of a sudden Flash Builder 4 is missing all kinds of fundamental things and is generating incorrect errors. I've had the same issue yesterday, where I fixed it by downloading a new Flex SDK and importing that into FB. I did this again, but this time it fixed nothing.
I don't think it's something I did, like removing critical references from the build path. The errors also appeared on projects I was not working on at the time. It occurs for ActionScript, Flex and Flex Library projects alike.
Update 3: Well, i've singled the problem down to a single piece of code, though a very simple one. I can make a new workspace in FB and things work ok, then screw the workspace up forever by adding this code to a project. All projects will have errors and closing or even removing the faulty project does not change this. Making another new workspace (without the faulty code) makes my projects compile again.
Link: http://www.the3rdage.net/files/2745/Main.as
(i've uploaded the file in case an odd character or encoding error causes the error)
Update 2: I've tried manual compiling with mxmlc, the same errors occur. It appears to be an SDK problem, not Flash Builder.
Update: I find this stack trace in the Flash Builder error log:
!ENTRY com.adobe.flexbuilder.project 4 43 2010-05-11 11:55:47.495
!MESSAGE Uncaught exception in compiler
!STACK 0
java.lang.NullPointerException
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:2592)
at macromedia.asc.parser.VariableBindingNode.evaluate(VariableBindingNode.java:64)
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:2233)
at macromedia.asc.parser.ListNode.evaluate(ListNode.java:44)
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:2578)
at macromedia.asc.parser.VariableDefinitionNode.evaluate(VariableDefinitionNode.java:48)
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:2310)
at macromedia.asc.parser.StatementListNode.evaluate(StatementListNode.java:60)
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:2503)
at macromedia.asc.parser.WithStatementNode.evaluate(WithStatementNode.java:44)
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:2310)
at macromedia.asc.parser.StatementListNode.evaluate(StatementListNode.java:60)
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:2891)
at macromedia.asc.parser.FunctionCommonNode.evaluate(FunctionCommonNode.java:106)
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:2905)
at macromedia.asc.parser.FunctionCommonNode.evaluate(FunctionCommonNode.java:106)
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:3643)
at macromedia.asc.parser.ClassDefinitionNode.evaluate(ClassDefinitionNode.java:106)
at macromedia.asc.semantics.ConstantEvaluator.evaluate(ConstantEvaluator.java:3371)
at macromedia.asc.parser.ProgramNode.evaluate(ProgramNode.java:80)
at flex2.compiler.as3.As3Compiler.analyze4(As3Compiler.java:709)
at flex2.compiler.CompilerAPI.analyze(CompilerAPI.java:3089)
at flex2.compiler.CompilerAPI.analyze(CompilerAPI.java:2977)
at flex2.compiler.CompilerAPI.batch2(CompilerAPI.java:528)
at flex2.compiler.CompilerAPI.batch(CompilerAPI.java:1274)
at flex2.compiler.CompilerAPI.compile(CompilerAPI.java:1496)
at flex2.tools.oem.Application.compile(Application.java:1188)
at flex2.tools.oem.Application.recompile(Application.java:1133)
at flex2.tools.oem.Application.compile(Application.java:819)
at flex2.tools.flexbuilder.BuilderApplication.compile(BuilderApplication.java:344)
at com.adobe.flexbuilder.multisdk.compiler.internal.ASApplicationBuilder$MyBuilder.mybuild(ASApplicationBuilder.java:276)
at com.adobe.flexbuilder.multisdk.compiler.internal.ASApplicationBuilder.build(ASApplicationBuilder.java:127)
at com.adobe.flexbuilder.multisdk.compiler.internal.ASBuilder.build(ASBuilder.java:190)
at com.adobe.flexbuilder.multisdk.compiler.internal.ASItemBuilder.build(ASItemBuilder.java:74)
at com.adobe.flexbuilder.project.compiler.internal.FlexProjectBuilder.buildItem(FlexProjectBuilder.java:480)
at com.adobe.flexbuilder.project.compiler.internal.FlexProjectBuilder.build(FlexProjectBuilder.java:306)
at com.adobe.flexbuilder.project.compiler.internal.FlexIncrementalBuilder.build(FlexIncrementalBuilder.java:157)
at org.eclipse.core.internal.events.BuildManager$2.run(BuildManager.java:627)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:170)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:201)
at org.eclipse.core.internal.events.BuildManager$1.run(BuildManager.java:253)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:256)
at org.eclipse.core.internal.events.BuildManager.basicBuildLoop(BuildManager.java:309)
at org.eclipse.core.internal.events.BuildManager.build(BuildManager.java:341)
at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBuildJob.java:140)
at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJob.java:238)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)
