Cannot link FLEX SDK 4 source code into Flex Project without errors

Steps to reproduce:
1. Create a new Flex or ActionScript project
2. Goto Project Properties window, select "Flex Build Path"
3. In "Source Path" tab click "Addo Folder..." button
4. Navigate to Flex 4 SDK source. ${FLEX_HOME}/frameworks/projects/framework/src

Actual Results:

Description Resource Path Location Type
1046: Type was not found or was not a compile-time constant: Number. MatrixUtil.as Test/[source path] src/mx/utils line 73 Flex Problem

Screenshot attached

Expected Results:

No errors or warrnigs. I am using SDK source in Flex Navigator for browsing into code

Workaround (if any):