Analyzer: case labels need some constraints checked

[DartBot]

This issue was originally filed by zundel@google.com

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main() {
  var x = 1;
  try {
    switch(x) {
      case 0:
      case 2:
      case "false": // should error here
    }
  } catch(var e) {}
}

Given a switch statement of the form
 * switch (e) { label11 … label1j1 case e1: s1 … labeln1 ..labelnjn case en: sn default: sn+1}
 * or the form switch (e) { label11 … label1j1 case e1: s1 … labeln1 ..labelnjn case en: sn},
 * it is a compile-time error if the expressions ek are not compile-time constants,
 * of type int or String, for all 1 <= k <= n.

See tests:
  co19/Language/11_Statements/08_Switch_A05_t01.dart
  co19/Language/11_Statements/08_Switch_A05_t02.dart
  co19/Language/11_Statements/08_Switch_A06_t01.dart
  co19/Language/11_Statements/08_Switch_A06_t02.dart
   ... and friends

[scheglov]

Set owner to @scheglov.
Added Accepted label.

[scheglov]

https://chromiumcodereview.appspot.com/10540114

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Added Started label.

[scheglov]

https://code.google.com/p/dart/source/detail?r=8573

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Added Fixed label.