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: 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)