Add a parseDartString method to pkg/analyzer.

pkg/analyzer_experimental/test/parse_compilation_unit_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.
+
+import 'package:analyzer_experimental/analyzer.dart';
+import 'package:unittest/unittest.dart';
+
+void main() {
+  test("parses a valid compilation unit successfully", () {
+    var unit = parseCompilationUnit("void main() => print('Hello, world!');");
+    expect(unit.toString(), equals("void main() => print('Hello, world!');"));
+  });
+
+  test("throws errors for an invalid compilation unit", () {
+    expect(() {
+      parseCompilationUnit("void main() => print('Hello, world!')",
+          name: 'test.dart');
+    }, throwsA(predicate((error) {
+      return error is AnalyzerErrorGroup &&
+          error.toString().contains("line 1 of test.dart");
+    })));
+  });
+
+  test("defaults to '<unknown source>' if no name is provided", () {
+    expect(() {
+      parseCompilationUnit("void main() => print('Hello, world!')");
+    }, throwsA(predicate((error) {
+      return error is AnalyzerErrorGroup &&
+          error.toString().contains("line 1 of <unknown source>");
+    })));
+  });
+}