Updated bundled libraries will not cause a recompilation of dependent files.

**What steps will reproduce the problem?**
1. Write a test dart program dependent on dart:core (e.g. String)
2. Compile test
3. Modify corelib (e.g. String)
4. Recompile test

**What is the expected output? What do you see instead?**
Expected: Files with dependencies recompiled
Actual: Zero delta compile
