Use backtracking when solving dependency constraints.

utils/tests/pub/install/pub_install_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS d.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 d.file.
 
 library pub_tests;
 
 import 'dart:io';
 
 import 'package:pathos/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
@@ skipping 37 matching lines @@
     schedulePub(args: ['install'],
         output: new RegExp(r"Dependencies installed!$"));
 
     d.dir(packagesPath, [
       d.dir("myapp_name", [
         d.file('foo.dart', 'main() => "foo";')
       ])
     ]).validate();
   });
 
-  integration('does not adds itself to the packages if it has no "lib" directory', () {
+  integration('does not add itself to the packages if it has no "lib" directory', () {
     // The symlink should use the name in the pubspec, not the name of the
     // directory.
     d.dir(appPath, [
       d.pubspec({"name": "myapp_name"}),
     ]).create();
 
     schedulePub(args: ['install'],
         output: new RegExp(r"Dependencies installed!$"));
 
     d.dir(packagesPath, [
@@ skipping 11 matching lines @@
       d.pubspec({"name": "myapp", "dependencies": {"foo": {"path": "../foo"}}})
     ]).create();
 
     schedulePub(args: ['install'],
         error: new RegExp(r'Warning: Package "foo" does not have a "lib" '
             'directory so you will not be able to import any libraries from '
             'it.'),
         output: new RegExp(r"Dependencies installed!$"));
   });
 
+  integration('reports a solver failure', () {
+    // myapp depends on foo and bar which both depend on baz with mismatched
+    // descriptions.
+    d.dir('deps', [
+      d.dir('foo', [
+        d.pubspec({"name": "foo", "dependencies": {
+          "baz": {"path": "../baz1"}
+        }})
+      ]),
+      d.dir('bar', [
+        d.pubspec({"name": "bar", "dependencies": {
+          "baz": {"path": "../baz2"}
+        }})
+      ]),
+      d.dir('baz1', [
+        d.libPubspec('baz', '0.0.0')
+      ]),
+      d.dir('baz2', [
+        d.libPubspec('baz', '0.0.0')
+      ])
+    ]).create();
+
+    d.dir(appPath, [
+      d.pubspec({"name": "myapp", "dependencies": {
+        "foo": {"path": "../deps/foo"},
+        "bar": {"path": "../deps/bar"}
+      }})
+    ]).create();
+
+    schedulePub(args: ['install'],
+        error: new RegExp(r"^Incompatible dependency descriptions on 'baz':"),
+        exitCode: 1);
+  });
+
   integration('does not warn if the root package lacks a "lib" directory', () {
     d.dir(appPath, [
       d.appPubspec([])
     ]).create();
 
     schedulePub(args: ['install'],
         error: new RegExp(r'^\s*$'),
         output: new RegExp(r"Dependencies installed!$"));
   });
 
@@ skipping 167 matching lines @@
           d.dir("packages", [
             d.dir("myapp", [
               d.file('foo.dart', 'main() => "foo";')
             ])
           ])
         ])
       ]).validate();
     });
   });
 }