Implement modular compilation

test/codegen/map_keys.dart

+// compile options: --source-map
+// TODO(jmesserly): more comprehensive strategy for testing the source map.
+// (this is used so we're covering it in at least one test)
+
 import 'dart:math' show Random;
 main() {
   // Uses a JS object literal
   print({ '1': 2, '3': 4, '5': 6 });
   // Uses array literal
   print({ 1: 2, 3: 4, 5: 6 });
   // Uses ES6 enhanced object literal
   print({ '1': 2, '${new Random().nextInt(2) + 2}': 4, '5': 6 });
   String x = '3';
   // Could use enhanced object literal if we knew `x` was not null
   print({ '1': 2, x: 4, '5': 6 });
   // Array literal
   print({ '1': 2, null: 4, '5': 6 });
 }