script compactor now names its bootstrap file based off the html entrypoint, not the dart file

lib/build/script_compactor.dart

Index: lib/build/script_compactor.dart
diff --git a/lib/build/script_compactor.dart b/lib/build/script_compactor.dart
index b3786f27b2d2bac9c96ecfde9f77e09c97cbd7d3..3492160c496626ffcaf8fe57c0a693d8e683bc22 100644
--- a/lib/build/script_compactor.dart
+++ b/lib/build/script_compactor.dart
@@ -104,8 +104,8 @@ class ScriptCompactor {
   /// Builds the bootstrap file and returns the path to it relative to
   /// [primaryInput].
   Asset _buildBootstrapFile(AssetId mainScript, Set<AssetId> importScripts) {
-    var bootstrapId = new AssetId(mainScript.package,
-        mainScript.path.replaceFirst('.dart', '.bootstrap.dart'));
+    var bootstrapId = new AssetId(primaryInput.package,
+        primaryInput.path.replaceFirst('.html', '.bootstrap.dart'));
 
     var buffer = new StringBuffer();
     buffer.writeln('library ${_libraryNameFor(bootstrapId)};');