Run pub with --trace (#12328)

See https://github.com/dart-lang/pub/issues/1714

packages/flutter_tools/lib/src/dart/pub.dart

@@ -81,12 +81,18 @@ typedef String MessageFilter(String message);
 /// Runs pub in 'batch' mode, forwarding complete lines written by pub to its
 /// stdout/stderr streams to the corresponding stream of this process, optionally
 /// applying filtering. The pub process will not receive anything on its stdin stream.
+///
+/// The `--trace` argument is passed to `pub` (by mutating the provided
+/// `arguments` list) unless `showTraceForErrors` is false.
 Future<Null> pub(List<String> arguments, {
   String directory,
   MessageFilter filter,
   String failureMessage: 'pub failed',
   @required bool retry,
+  bool showTraceForErrors: true,
 }) async {
+  if (showTraceForErrors)
+    arguments.insert(0, '--trace');
   int attempts = 0;
   int duration = 1;
   int code;

packages/flutter_tools/test/commands/packages_test.dart

@@ -82,10 +82,11 @@ void main() {
     testUsingContext('test', () async {
       await createTestCommandRunner(new PackagesCommand()).run(<String>['packages', 'test']);
       final List<String> commands = mockProcessManager.commands;
-      expect(commands, hasLength(3));
+      expect(commands, hasLength(4));
       expect(commands[0], matches(r'dart-sdk[\\/]bin[\\/]pub'));
-      expect(commands[1], 'run');
-      expect(commands[2], 'test');
+      expect(commands[1], '--trace');
+      expect(commands[2], 'run');
+      expect(commands[3], 'test');
     }, overrides: <Type, Generator>{
       ProcessManager: () => mockProcessManager,
       Stdio: () => mockStdio,