Nicer message when attempting to use Observatory from dart_bootstrap, dart_precompiled_runtime or a…

runtime/bin/vmservice/server.dart

Index: runtime/bin/vmservice/server.dart
diff --git a/runtime/bin/vmservice/server.dart b/runtime/bin/vmservice/server.dart
index fb272607f56cdd08ebc904fa0e908ddb90f7b3a4..a0446ad222a02f5e3ac7489f510a44acbff88931 100644
--- a/runtime/bin/vmservice/server.dart
+++ b/runtime/bin/vmservice/server.dart
@@ -296,6 +296,12 @@ class Server {
       return;
     }
 
+    if (assets == null) {
+      request.response.headers.contentType = ContentType.TEXT;
+      request.response.write("This VM was built without the Observatory UI.");
+      request.response.close();
+      return;
+    }
     Asset asset = assets[path];
     if (asset != null) {
       // Serving up a static asset (e.g. .css, .html, .png).