Don't declare the HTML output of the PubServeTransformer.

This output won't always be generated, and if we always declare it
"pub serve" will produce unsightly error messages when it would
conflict if generated.

See #84

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1097383002

CHANGELOG.md

@@ -4,6 +4,9 @@
 
 * Preserve the stack traces for load errors in isolates and iframes.
 
+* Stop `pub serve` from emitting a duplicate-asset error for tests with custom
+  HTML files.
+
 ### 0.12.0-beta.10
 
 * Fix running browser tests in subdirectories.

lib/pub_serve.dart

@@ -22,7 +22,6 @@ class PubServeTransformer extends Transformer implements DeclaringTransformer {
     var id = transform.primaryId;
     transform.declareOutput(id.addExtension('.vm_test.dart'));
     transform.declareOutput(id.addExtension('.browser_test.dart'));
-    transform.declareOutput(id.changeExtension('.html'));
   }
 
   Future apply(Transform transform) {