Make type inference handle overloaded arithmetic operations.

pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
index f08da1ced68551c02d24d30a471c56a7a7205595..3165a2449736cdd067c59477573e858cd0917daf 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
@@ -158,6 +158,18 @@ class TypeSchemaEnvironment extends TypeEnvironment {
     return const DynamicType();
   }
 
+  @override
+  DartType getTypeOfOverloadedArithmetic(DartType type1, DartType type2) {
+    // TODO(paulberry): this matches what is defined in the spec.  It would be
+    // nice if we could change kernel to match the spec and not have to
+    // override.
+    if (type1 == intType) {
+      if (type2 == intType) return type2;
+      if (type2 == doubleType) return type2;
+    }
+    return numType;
+  }
+
   /// Infers a generic type, function, method, or list/map literal
   /// instantiation, using the downward context type as well as the argument
   /// types if available.