Move source maps package to the dart repo, so it can be used by other internal

pkg/source_maps/test/builder_test.dart

+// Copyright (c) 2013, 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.
+
+library test.source_maps_test;
+
+import 'dart:json' as json;
+import 'package:unittest/unittest.dart';
+import 'package:source_maps/source_maps.dart';
+import 'common.dart';
+
+main() {
+  test('builder - with span', () {
+    var map = (new SourceMapBuilder()
+        ..addSpan(inputVar1, outputVar1)
+        ..addSpan(inputFunction, outputFunction)
+        ..addSpan(inputVar2, outputVar2)
+        ..addSpan(inputExpr, outputExpr))
+        .build(output.url);
+    expect(map, equals(EXPECTED_MAP));
+  });
+
+  test('builder - with location', () {
+    var str = (new SourceMapBuilder()
+        ..addLocation(inputVar1.start, outputVar1.start, 'longVar1')
+        ..addLocation(inputFunction.start, outputFunction.start, 'longName')
+        ..addLocation(inputVar2.start, outputVar2.start, 'longVar2')
+        ..addLocation(inputExpr.start, outputExpr.start, null))
+        .toJson(output.url);
+    expect(str, json.stringify(EXPECTED_MAP));
+  });
+}