Optimize more megamorphic dispatch sites

pkg/analyzer/lib/instrumentation/instrumentation.dart

Index: pkg/analyzer/lib/instrumentation/instrumentation.dart
diff --git a/pkg/analyzer/lib/instrumentation/instrumentation.dart b/pkg/analyzer/lib/instrumentation/instrumentation.dart
index 3e93fb738489579ad6c47188e5b2cda0655a99f6..eaf95ba56e87dcb58533c1c65fffc63c39c11de9 100644
--- a/pkg/analyzer/lib/instrumentation/instrumentation.dart
+++ b/pkg/analyzer/lib/instrumentation/instrumentation.dart
@@ -320,9 +320,10 @@ class InstrumentationService {
   String _join(List<String> fields) {
     StringBuffer buffer = new StringBuffer();
     buffer.write(_timestamp);
-    for (String field in fields) {
+    int length = fields.length;
+    for (int i = 0; i < length; i++) {
       buffer.write(':');
-      _escape(buffer, field);
+      _escape(buffer, fields[i]);
     }
     return buffer.toString();
   }