Fix Observatory recompiles on fresh build

runtime/observatory/BUILD.gn

Index: runtime/observatory/BUILD.gn
diff --git a/runtime/observatory/BUILD.gn b/runtime/observatory/BUILD.gn
index c0903ea49725f1104cf9ef5e85f1b2189194551f..3943253ca45cb305c75f978f8205e03b9fe49a29 100644
--- a/runtime/observatory/BUILD.gn
+++ b/runtime/observatory/BUILD.gn
@@ -121,7 +121,7 @@ action("pub_build_observatory") {
   ]
 
   outputs = [
-    "$root_out_dir/observatory/build/web/index.html.polymer.bootstrap.dart.js",
+    "$root_out_dir/observatory/build/web/main.dart.js",
   ]
 }
 
@@ -134,7 +134,7 @@ action("deploy_observatory") {
 
   inputs = [
     script,
-    "$root_out_dir/observatory/build/web/index.html.polymer.bootstrap.dart.js",
+    "$root_out_dir/observatory/build/web/main.dart.js",
   ]
 
   args = [
@@ -148,7 +148,7 @@ action("deploy_observatory") {
   ]
 
   outputs = [
-    "$root_out_dir/observatory/deployed/web/index.html.polymer.bootstrap.dart.js",
+    "$root_out_dir/observatory/deployed/web/main.dart.js",
   ]
 }
 
@@ -161,7 +161,7 @@ action("archive_observatory") {
 
   inputs = [
     script,
-    "$root_out_dir/observatory/deployed/web/index.html.polymer.bootstrap.dart.js",
+    "$root_out_dir/observatory/deployed/web/main.dart.js",
   ]
 
   args = [