improve handling of private methods in polymer code generation

[sigmundch]

See: https://code.google.com/p/dart/issues/detail?id=18481

Options:
(a) give better error messages in cases like _highlightChanged
(b) add support for private methods. This would require:
  - switching smoke to use strings instead of symbols (#_foo is not supported)
  - switching codegeneration to generate code on each library (requires issue #18489), we then can generate the getters for these private symbols in the context of the same library

[sigmundch]

Marked this as blocking #18481.

[sigmundch]

Issue #13881 has been merged into this issue.

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cc @jmesserly.

[sigmundch]

Removed Type-Defect label.
Added Type-Enhancement label.

[sigmundch]

Added this to the Later milestone.

[jmesserly]

Added Pkg-Polymer label.

[jmesserly]

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[jmesserly]

Removed Library-PolymerBuild label.

[jmesserly]

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[sigmundch]

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[sigmundch]

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[sigmundch]

Added Polymer-P-2 label.

[sigmundch]

Removed this from the Later milestone.

[sigmundch]

Removed Polymer-P-2 label.
Added Polymer-Milestone-Later label.

[sigmundch]

Removed Polymer-Milestone-Later label.
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