fix some errors in our SDK, mostly around numbers

lib/src/checker/rules.dart

 // Copyright (c) 2015, 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 dev_compiler.src.checker.rules;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/resolver.dart';
 
@@ skipping 19 matching lines @@
   bool isGroundType(DartType t) => true;
   // TODO(vsm): The default implementation is not ignoring the return type,
   // only the restricted override is.
   bool isFunctionSubTypeOf(FunctionType f1, FunctionType f2,
           {bool fuzzyArrows: true, bool ignoreReturn: false}) =>
       isSubTypeOf(f1, f2);
 
   bool isBoolType(DartType t) => t == provider.boolType;
   bool isDoubleType(DartType t) => t == provider.doubleType;
   bool isIntType(DartType t) => t == provider.intType;
-  bool isNumType(DartType t) => t == provider.intType.superclass;
+  bool isNumType(DartType t) => t == provider.numType;
+
+  /// Returns true if this is any kind of object represented by `Number` in JS.
+  ///
+  /// In practice, this is 4 types: num, int, double, and JSNumber.
+  ///
+  /// JSNumber is the type that actually "implements" all numbers, hence it's
+  /// a subtype of int and double (and num). It's in our "dart:_interceptors".
+  bool isNumberInJS(DartType t) => isSubTypeOf(t, provider.numType);
+
   bool isStringType(DartType t) => t == provider.stringType;
   bool isNonNullableType(DartType t) => false;
   bool maybeNonNullableType(DartType t) => false;
 
   StaticInfo checkAssignment(Expression expr, DartType t);
 
   DartType getStaticType(Expression expr) => expr.staticType;
 
   /// Given a type t, if t is an interface type with a call method
   /// defined, return the function type for the call method, otherwise
@@ skipping 787 matching lines @@
     var entries = e.entries;
     bool inferEntry(MapLiteralEntry entry) {
       return _inferExpression(entry.key, kType, errors) &&
           _inferExpression(entry.value, vType, errors);
     }
     var b = entries.every(inferEntry);
     if (b) annotateMapLiteral(e, targs);
     return b;
   }
 }