In non-strong mode, don't infer the type of for-in loop variables.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index 84ceb23eefefc33a9a408f54d582d2d890b3f8c9..0e2a3449a75b71259d4f9bd428e5b7c4e939754d 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -736,7 +736,7 @@ class KernelForInStatement extends ForInStatement implements KernelStatement {
     KernelVariableDeclaration variable;
     if (_declaresVariable) {
       variable = this.variable;
-      if (variable._implicitlyTyped) {
+      if (inferrer.strongMode && variable._implicitlyTyped) {
         typeNeeded = true;
         // TODO(paulberry): In this case, should the context be `Iterable<?>`?
       } else {