Observatory strong mode fix: Avoid a String/Uri type mismatch.

Change-Id: Ie4871b7467bd00e594f7df397909690461fe358a
Reviewed-on: https://dart-review.googlesource.com/56702
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
diff --git a/runtime/observatory/lib/src/elements/script_inset.dart b/runtime/observatory/lib/src/elements/script_inset.dart
index 5f59f6f..6937c9e 100644
--- a/runtime/observatory/lib/src/elements/script_inset.dart
+++ b/runtime/observatory/lib/src/elements/script_inset.dart
@@ -433,9 +433,9 @@
       }
     }
     if (targetUri.scheme == 'package') {
-      targetUri = "packages/${targetUri.path}";
+      var targetUriString = "packages/${targetUri.path}";
       for (M.Library l in script.isolate.libraries) {
-        if (targetUri.toString() == l.uri) {
+        if (targetUriString == l.uri) {
           return l;
         }
       }