test(http): disable a flaky expectation

The error message varies across browsers:

    Dartium -> "Unexpected character"
    Dart2JS:
      - Chrome -> "Unexpected token"
      - Firefox -> "unexpected character"
      - IE -> "Invalid character"

Author: vicb

test/core_dom/http_spec.dart

@@ -1371,10 +1371,13 @@ void main() {
               http.get('/url').then((_) {
                 callbackCalled = true;
               }, onError: (e,s) {
-                // Dartium throws "Unexpected character"
-                // dart2js/Chrome throws "Unexpected token"
-                // dart2js/Firefox throw "unexpected character"
-                expect('$e').toContain('nexpected');
+                // Dartium -> "Unexpected character"
+                // dart2js:
+                // - Chrome -> "Unexpected token"
+                // - Firefox -> "unexpected character"
+                // - IE -> "Invalid character"
+                // Commented out as this expectation is not robust !
+                // expect('$e', unit.).toContain('nexpected');
                 onErrorCalled = true;
               });
               flush();