Support debugging polymer apps with polymer.js elements

[sigmundch]

Some things that would be great to have:
 * stack traces where you can see JS and Dart
 * switching between js and dart in the console

(additional context in https://code.google.com/p/dart/issues/detail?id=19203#c3)

[jmesserly]

fixed call stacks is filed as https://code.google.com/p/dart/issues/detail?id=19537
switching contexts is filed as https://code.google.com/p/dart/issues/detail?id=19811

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Added Duplicate label.
Marked as being merged into #19537.