Fix analysis warnings.

lib/src/util/string_literal_iterator.dart

Index: lib/src/util/string_literal_iterator.dart
diff --git a/lib/src/util/string_literal_iterator.dart b/lib/src/util/string_literal_iterator.dart
index 67fffbb58a76cab577f568504fea3ca96e972fa2..59f0372e376a755644de8af36759ac9ca42a17fd 100644
--- a/lib/src/util/string_literal_iterator.dart
+++ b/lib/src/util/string_literal_iterator.dart
@@ -88,7 +88,7 @@ class StringLiteralIterator extends Iterator<int> {
     } else {
       assert(literal is AdjacentStrings);
 
-      for (var string in literal.strings) {
+      for (var string in (literal as AdjacentStrings).strings) {
         if (string is StringInterpolation) {
           throw new ArgumentError("Can't iterate over an interpolated string.");
         }