Fix 29264: remove access to the network on source-map unit test

tests/compiler/dart2js/source_map_pub_build_validity_test.dart

Index: tests/compiler/dart2js/source_map_pub_build_validity_test.dart
diff --git a/tests/compiler/dart2js/source_map_pub_build_validity_test.dart b/tests/compiler/dart2js/source_map_pub_build_validity_test.dart
index 5eaff5b7256f96d7506c58cf949799ecafee32d2..9879f183299befde6b641bfed0928741c9814c58 100644
--- a/tests/compiler/dart2js/source_map_pub_build_validity_test.dart
+++ b/tests/compiler/dart2js/source_map_pub_build_validity_test.dart
@@ -23,9 +23,16 @@ void main() {
           path.fromUri(Platform.script), '../../../../sdk/bin/pub${ext}'));
       String file = path.join(tmpDir.path, 'build/web/sunflower.dart.js');
 
-      print("Running '$command get' from '${tmpDir}'.");
-      ProcessResult getResult =
-          await Process.run(command, ['get'], workingDirectory: tmpDir.path);
+      // sunflower/pubspec.yaml only depends on package:browser for Dartium, we
+      // override the file to remove this dependency and make pub-get --offline
+      // trivially succeed.
+      print("Overriding '${tmpDir.path}/pubspec.yaml'.");
+      new File(path.join(tmpDir.path, "pubspec.yaml"))
+          .writeAsStringSync(_newPubspec);
+
+      print("Running '$command get --offline' from '${tmpDir}'.");
+      ProcessResult getResult = await Process.run(command, ['get', '--offline'],
+          workingDirectory: tmpDir.path);
       print(getResult.stdout);
       print(getResult.stderr);
       Expect.equals(0, getResult.exitCode, 'Unexpected exitCode from pub get');
@@ -44,3 +51,9 @@ void main() {
     }
   });
 }
+
+String _newPubspec = '''
+# Generated by test
+name: sunflower
+version: 0.0.0
+''';