Fix analysis warnings.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1041453002

lib/src/backend/platform_selector/evaluator.dart

@@ -4,8 +4,6 @@
 
 library unittest.backend.platform_selector.evaluator;
 
-import 'package:source_span/source_span.dart';
-
 import '../operating_system.dart';
 import '../test_platform.dart';
 import 'ast.dart';

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.");
         }