Use a non-null TypeInferrer even in non-strong mode.

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.body_builder;
 
 import '../fasta_codes.dart'
     show FastaMessage, codeExpectedButGot, codeExpectedFunctionBody;
 
 import '../parser/parser.dart' show FormalParameterType, optional;
@@ skipping 428 matching lines @@
 
   @override
   void endInitializers(int count, Token beginToken, Token endToken) {
     debugEvent("Initializers");
   }
 
   @override
   void finishFunction(
       FormalParameters formals, AsyncMarker asyncModifier, Statement body) {
     debugEvent("finishFunction");
-    _typeInferrer?.inferBody(body, uri);
+    _typeInferrer.inferBody(body, uri);
     KernelFunctionBuilder builder = member;
     builder.body = body;
     if (formals?.optional != null) {
       Iterator<FormalParameterBuilder> formalBuilders =
           builder.formals.skip(formals.required.length).iterator;
       for (VariableDeclaration parameter in formals.optional.formals) {
         bool hasMore = formalBuilders.moveNext();
         assert(hasMore);
         VariableDeclaration realParameter = formalBuilders.current.target;
         Expression initializer = parameter.initializer ?? new NullLiteral();
@@ skipping 2614 matching lines @@
   } else if (node is PrefixBuilder) {
     return node.name;
   } else if (node is ThisAccessor) {
     return node.isSuper ? "super" : "this";
   } else if (node is FastaAccessor) {
     return node.plainNameForRead;
   } else {
     return internalError("Unhandled: ${node.runtimeType}");
   }
 }