Improve recovery for unterminated interpolated expressions

[jtmcdole]

When processing the following:
  main() {
    print("${42");
    foo();
    bar();
    baz();
    print("end");
  }

The scanner will keep scanning as in an embedded expression till EOS (and then insert the STRING_EMBED_EXP_END + ILLEGAL). The part trailing ${42, ");", will be lost from the token stream.

Parsing errors consumes to much and doesn't known about the single-line string vs multi-line string state. The expression, 42, is returned while parsing the string and the ILLEGAL token is consumed and the rest of the program is encoded as part of the string:
    DartFunction
      DartBlock
        DartExprStmt
          DartUnqualifiedInvocation
            DartIdentifier (print)
            DartStringInterpolation
              DartStringLiteral ("")
              DartStringLiteral ("")
              DartStringLiteral ("")
              DartStringLiteral ("")
              DartStringLiteral ("")
              DartStringLiteral ("")
              DartStringLiteral ("")
              DartIntegerLiteral (42)
              DartSyntheticErrorExpression( (foo))
              DartSyntheticErrorExpression( (bar))
              DartSyntheticErrorExpression( (baz))
              DartSyntheticErrorExpression( (print))
              DartSyntheticErrorExpression( (end))

[kasperl]

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

[danrubel]

Added this to the M1 milestone.

[bwilkerson]

Removed this from the M1 milestone.
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

---

Added Editor-AnalysisEngine label.

[bwilkerson]

Analysis engine does a better job, but produces too many errors.

---

Set owner to @bwilkerson.
Removed AnalysisEngine, Editor-AnalysisEngine labels.
Changed the title to: "Improve recovery for unterminated interpolated expressions".

[bwilkerson]

Added Analyzer-Recovery label.

[bwilkerson]

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

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[kasperl]

Removed Oldschool-Milestone-Later label.

[bwilkerson]

Issue #2168 has been merged into this issue.