Detect and warn about missing scripts (rather than fail during the build)

pkg/polymer/lib/src/build/script_compactor.dart

Index: pkg/polymer/lib/src/build/script_compactor.dart
diff --git a/pkg/polymer/lib/src/build/script_compactor.dart b/pkg/polymer/lib/src/build/script_compactor.dart
index 0af62005d1065a4e596edd0373b63b93c616331d..b90dfe1ca2c1ad11fdb746e646667aaf207bf5a0 100644
--- a/pkg/polymer/lib/src/build/script_compactor.dart
+++ b/pkg/polymer/lib/src/build/script_compactor.dart
@@ -139,12 +139,8 @@ class _ScriptCompactor extends PolymerTransformer {
   /// Emits the main HTML and Dart bootstrap code for the application. If there
   /// were not Dart entry point files, then this simply emits the original HTML.
   Future _emitNewEntrypoint(_) {
-    if (entryLibraries.isEmpty) {
-      // We didn't find code, nothing to do.
-      transform.addOutput(transform.primaryInput);
-      return null;
-    }
-
+    // If we don't find code, there is nothing to do.
+    if (entryLibraries.isEmpty) return null;
     return _initResolver()
         .then(_extractUsesOfMirrors)
         .then(_emitFiles)
@@ -157,10 +153,18 @@ class _ScriptCompactor extends PolymerTransformer {
   /// [entryLibraries], then use it to initialize the [recorder] (for import
   /// resolution) and to resolve specific elements (for analyzing the user's
   /// code).
-  Future _initResolver() => resolvers.get(transform, entryLibraries).then((r) {
-    resolver = r;
-    types = new _ResolvedTypes(resolver);
-  });
+  Future _initResolver() {
+    // We include 'polymer.dart' to simplify how we do resolution below. This
+    // way we can assume polymer is there, even if the user didn't include an
+    // import to it. If not, the polymer build will fail with an error when
+    // trying to create _ResolvedTypes below.
+    var libsToLoad = [new AssetId('polymer', 'lib/polymer.dart')]
+        ..addAll(entryLibraries);
+    return resolvers.get(transform, libsToLoad).then((r) {
+      resolver = r;
+      types = new _ResolvedTypes(resolver);
+    });
+  }
 
   /// Inspects the entire program to find out anything that polymer accesses
   /// using mirrors and produces static information that can be used to replace
@@ -345,15 +349,21 @@ class _ScriptCompactor extends PolymerTransformer {
     generator.writeTopLevelDeclarations(code);
     code.writeln('\nvoid main() {');
     generator.writeInitCall(code);
-    code.writeln('  startPolymer([');
+    code.write('  startPolymer([');
 
     // Include initializers to switch from mirrors_loader to static_loader.
-    for (var init in initializers) {
-      var initCode = init.asCode(prefixes[init.assetId]);
-      code.write("      $initCode,\n");
+    if (!initializers.isEmpty) {
+      code.writeln();
+      for (var init in initializers) {
+        var initCode = init.asCode(prefixes[init.assetId]);
+        code.write("      $initCode,\n");
+      }
+      code.writeln('    ]);');
+    } else {
+      logger.warning(NO_INITIALIZERS_ERROR);
+      code.writeln(']);');
     }
-    code..writeln('    ]);')
-        ..writeln('}');
+    code.writeln('}');
     transform.addOutput(new Asset.fromString(bootstrapId, code.toString()));
 
 
@@ -404,6 +414,13 @@ library app_bootstrap;
 import 'package:polymer/polymer.dart';
 """;
 
+const NO_INITIALIZERS_ERROR =
+    'No polymer initializers were found. Make sure to either '
+    'annotate your polymer elements with @CustomTag or include a '
+    'top level method annotated with @initMethod that registers your '
+    'elements. Both annotations are defined in the polymer library ('
+    'package:polymer/polymer.dart).';
+
 /// An html visitor that:
 ///   * finds all polymer expressions and records the getters and setters that
 ///     will be needed to evaluate them at runtime.