pkg/matcher: Small tweaks to improve analysis

lib/src/core_matchers.dart

Index: lib/src/core_matchers.dart
diff --git a/lib/src/core_matchers.dart b/lib/src/core_matchers.dart
index a3d1f5750dcfeea6736b26a4b8073a82ee31de0a..61585bcf2c8a822fd2eb67856973bd036eb9a980 100644
--- a/lib/src/core_matchers.dart
+++ b/lib/src/core_matchers.dart
@@ -302,8 +302,8 @@ class _StringEqualsMatcher extends Matcher {
       int minLength = escapedItem.length < escapedValue.length
           ? escapedItem.length
           : escapedValue.length;
-      int start;
-      for (start = 0; start < minLength; start++) {
+      var start = 0;
+      for (; start < minLength; start++) {
         if (escapedValue.codeUnitAt(start) != escapedItem.codeUnitAt(start)) {
           break;
         }