Add support for sync* and yield

tests/language/sync_generator1_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Simple test program for sync* generator functions.
+
+import "package:expect/expect.dart";
+
+sum10() sync* {
+  var s = 0;
+  for (var k = 1; k <= 10; k++) {
+    s += k;
+    yield s;
+  }
+}
+
+class Range {
+  int start, end;
+  Range(this.start, this.end);
+  elements() sync* {
+    var e = start;
+    while (e <= end) yield e++;
+  }
+
+  get yield sync* {  // yield is a legal member name here.
+    var e = start;
+    while (e <= end) yield e++;
+  }
+}
+
+get sync sync* {  // sync is a legal identifier.
+  yield "sync";
+}
+
+einsZwei() sync* {
+  yield* 1;
+  yield* [2, 3];
+  yield* [];
+  yield 5;
+  yield [6];
+}
+
+main() {
+  var sums = sum10();
+  print(sums);
+  Expect.isTrue(sums is Iterable);
+  Expect.equals(10, sums.length);
+  Expect.equals(1, sums.first);
+  Expect.equals(55, sums.last);
+  var q = "";
+  for (var n in sums.take(3)) {
+    q += "$n ";
+  }
+  Expect.equals("1 3 6 ", q);
+
+  var r = new Range(10, 12);
+  var elems1 = r.elements();
+  print(elems1);
+  var elems2 = r.yield;
+  print(elems2);
+  // Walk the elements of each iterable and compare them.
+  var i = elems1.iterator;
+  Expect.isTrue(i is Iterator);
+  elems2.forEach((e) {
+    Expect.isTrue(i.moveNext());
+    Expect.equals(e, i.current);
+  });
+
+  print(sync);
+  Expect.equals("sync", sync.single);
+
+  print(einsZwei());
+  Expect.equals("(1, 2, 3, 5, [6])", einsZwei().toString());
+}