Improve pub's pubspec error messages.

sdk/lib/_internal/pub/test/test_pub.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.
 
 /// Test infrastructure for testing pub.
 ///
 /// Unlike typical unit tests, most pub tests are integration tests that stage
 /// some stuff on the file system, run pub, and then validate the results. This
 /// library provides an API to build tests like that.
 library test_pub;
@@ skipping 326 matching lines @@
 
 /// Schedules an invocation of pub [command] and validates that it completes
 /// in an expected way.
 ///
 /// By default, this validates that the command completes successfully and
 /// understands the normal output of a successful pub command. If [warning] is
 /// given, it expects the command to complete successfully *and* print
 /// [warning] to stderr. If [error] is given, it expects the command to *only*
 /// print [error] to stderr. [output], [error], and [warning] may be strings,
 /// [RegExp]s, or [Matcher]s.
+///
+/// If [exitCode] is given, expects the command to exit with that code.
 // TODO(rnystrom): Clean up other tests to call this when possible.
 void pubCommand(RunCommand command,
-    {Iterable<String> args, output, error, warning}) {
+    {Iterable<String> args, output, error, warning, int exitCode}) {
   if (error != null && warning != null) {
     throw new ArgumentError("Cannot pass both 'error' and 'warning'.");
   }
 
   var allArgs = [command.name];
   if (args != null) allArgs.addAll(args);
 
   if (output == null) output = command.success;
 
-  var exitCode = null;
-  if (error != null) exitCode = 1;
+  if (error != null && exitCode == null) exitCode = 1;
 
   // No success output on an error.
   if (error != null) output = null;
   if (warning != null) error = warning;
 
   schedulePub(args: allArgs, output: output, error: error, exitCode: exitCode);
 }
 
-void pubGet({Iterable<String> args, output, error, warning}) {
+void pubGet({Iterable<String> args, output, error, warning, int exitCode}) {
   pubCommand(RunCommand.get, args: args, output: output, error: error,
-      warning: warning);
+      warning: warning, exitCode: exitCode);
 }
 
-void pubUpgrade({Iterable<String> args, output, error, warning}) {
+void pubUpgrade({Iterable<String> args, output, error, warning, int exitCode}) {
   pubCommand(RunCommand.upgrade, args: args, output: output, error: error,
-      warning: warning);
+      warning: warning, exitCode: exitCode);
 }
 
 /// Defines an integration test.
 ///
 /// The [body] should schedule a series of operations which will be run
 /// asynchronously.
 void integration(String description, void body()) =>
   _integration(description, body, test);
 
 /// Like [integration], but causes only this test to run.
@@ skipping 534 matching lines @@
         _lastMatcher.matches(item.last, matchState);
   }
 
   Description describe(Description description) {
     return description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }
 
 /// A [StreamMatcher] that matches multiple lines of output.
 StreamMatcher emitsLines(String output) => inOrder(output.split("\n"));