Fix a bug in pubspec parsing.

We didn't properly check for no sources in a dependency, which caused a
crash for some invalid pubspecs.

Closes #1348

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1417993002 .

lib/src/pubspec.dart

@@ -463,6 +463,8 @@ class Pubspec {
         var sourceNames = spec.keys.toList();
         if (sourceNames.length > 1) {
           _error('A dependency may only have one source.', specNode.span);
+        } else if (sourceNames.isEmpty) {
+          _error('A dependency must contain a source.', specNode.span);
         }
 
         sourceName = sourceNames.single;

test/pubspec_test.dart

@@ -229,6 +229,14 @@ dependencies:
 ''', (pubspec) => pubspec.dependencies);
     });
 
+    test("throws if there's no source", () {
+      expectPubspecException('''
+dependencies:
+  foo:
+    version: 1.2.3
+''', (pubspec) => pubspec.dependencies);
+    });
+
     test("throws if 'name' is not a string", () {
       expectPubspecException('name: [not, a, string]',
           (pubspec) => pubspec.name);