dartc parser resiliency on library directives

[DartBot]

This issue was originally filed by zundel@google.com

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Write error, out but keep parsing if ; missing from end of #source, #library, or #import stmt

eg.

import("foo")

import("bar")

[kasperl]

Removed Area-Compiler label.
Added Area-Analyzer label.

[bwilkerson]

Added this to the Later milestone.

[bwilkerson]

Removed this from the Later milestone.
Added this to the M3 milestone.

[scheglov]

Removed this from the M3 milestone.
Added this to the Later milestone.
Added AnalysisEngine label.

[DartBot]

This comment was originally written by amouravski@google.com

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Added Editor-AnalysisEngine label.

[bwilkerson]

Removed AnalysisEngine, Editor-AnalysisEngine labels.

[scheglov]

I've validated that this is not an issue with the new analyzer.

import 'dart:math'
import 'dart:io'

main() {
  print(PI);
  File f = null;
}

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Removed this from the Later milestone.
Added this to the M5 milestone.
Added AssumedStale label.