Lazily throw pubspec parsing exceptions.

sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart

Index: sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart
diff --git a/sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart b/sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart
index e9d2d35452f87f98e005fc7ee28f5c5db7a14667..d43407cf10fa372c1d263e58242357d3c35604ef 100644
--- a/sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart
+++ b/sdk/lib/_internal/pub/lib/src/validator/pubspec_field.dart
@@ -39,21 +39,41 @@ class PubspecFieldValidator extends Validator {
       }
     }
 
-    var homepage = pubspec.fields['homepage'];
-    if (homepage == null) {
-      errors.add('Your pubspec.yaml is missing a "homepage" field.');
-    }
+    _validateFieldExists('description');
+    _validateFieldExists('version');
+    _validateFieldExists('homepage');
+    _validateFieldUrl('homepage');
+    _validateFieldUrl('documentation');
 
-    var description = pubspec.fields['description'];
-    if (description == null) {
-      errors.add('Your pubspec.yaml is missing a "description" field.');
+    // Pubspec errors are detected lazily, so we make sure there aren't any
+    // here.
+    for (var error in pubspec.allErrors) {
+      errors.add('In your pubspec.yaml, ${error.message}');
     }
 
-    var version = pubspec.fields['version'];
-    if (version == null) {
-      errors.add('Your pubspec.yaml is missing a "version" field.');
+    return new Future.value();
+  }
+
+  void _validateFieldExists(String field) {
+    if (entrypoint.root.pubspec.fields[field] != null) return;
+    errors.add('Your pubspec.yaml is missing a "$field" field.');
+  }
+
+  /// Adds an error if the given [url] for [field] is invalid.
+  void _validateFieldUrl(String field) {
+    var url = entrypoint.root.pubspec.fields[field];
+    if (url == null) return;
+
+    if (url is! String) {
+      errors.add('Your pubspec.yaml\'s "$field" field must be a string, but '
+          'it was "$url".');
+      return;
     }
 
-    return new Future.value();
+    var goodScheme = new RegExp(r'^https?:');
+    if (!goodScheme.hasMatch(url)) {
+      errors.add('Your pubspec.yaml\'s "$field" field must be an "http:" or '
+          '"https:" URL, but it was "$url".');
+    }
   }
 }