Add type inference for complex assignments whose LHS is an index expression.

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

Index: pkg/front_end/lib/src/fasta/kernel/body_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index d7a3571bb9009d3c9f591dc004fb999cc6fe1c3a..5a7c0d81dea82c322cd954fec9950500acd06eda 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -747,8 +747,8 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
     VariableDeclaration variable = new VariableDeclaration.forValue(a);
     push(makeLet(
         variable,
-        new ConditionalExpression(buildIsNull(new VariableGet(variable)), b,
-            new VariableGet(variable), const DynamicType())));
+        new KernelConditionalExpression(buildIsNull(new VariableGet(variable)),
+            b, new VariableGet(variable))));
   }
 
   /// Handle `a?.b(...)`.
@@ -2859,12 +2859,11 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
       VariableDeclaration variable = new VariableDeclaration.forValue(receiver);
       return makeLet(
           variable,
-          new ConditionalExpression(
+          new KernelConditionalExpression(
               buildIsNull(new VariableGet(variable)),
               new NullLiteral(),
               new MethodInvocation(new VariableGet(variable), name, arguments)
-                ..fileOffset = offset,
-              const DynamicType()));
+                ..fileOffset = offset));
     } else {
       return new KernelMethodInvocation(receiver, name, arguments,
           isImplicitCall: isImplicitCall)