Issue 25914. Fix for compound assignments like 'T += T'.

pkg/analyzer/lib/src/task/strong/checker.dart

Index: pkg/analyzer/lib/src/task/strong/checker.dart
diff --git a/pkg/analyzer/lib/src/task/strong/checker.dart b/pkg/analyzer/lib/src/task/strong/checker.dart
index 3e6f6f52196978450f264947e8b879ab49b9179a..6b0049bf9bc941236254dd786cadd3158578e823 100644
--- a/pkg/analyzer/lib/src/task/strong/checker.dart
+++ b/pkg/analyzer/lib/src/task/strong/checker.dart
@@ -622,8 +622,8 @@ class CodeChecker extends RecursiveAstVisitor {
       var staticInfo;
       var rhsType = _getStaticType(expr.rightHandSide);
       var lhsType = _getStaticType(expr.leftHandSide);
-      var returnType = _specializedBinaryReturnType(
-          op, lhsType, rhsType, functionType.returnType);
+      var returnType = rules.refineBinaryExpressionType(
+          typeProvider, lhsType, op, rhsType, functionType.returnType);
 
       if (!rules.isSubtypeOf(returnType, lhsType)) {
         final numType = typeProvider.numType;
@@ -921,32 +921,6 @@ class CodeChecker extends RecursiveAstVisitor {
       CoercionInfo.set(info.node, info);
     }
   }
-
-  DartType _specializedBinaryReturnType(
-      TokenType op, DartType t1, DartType t2, DartType normalReturnType) {
-    // This special cases binary return types as per 16.26 and 16.27 of the
-    // Dart language spec.
-    switch (op) {
-      case TokenType.PLUS:
-      case TokenType.MINUS:
-      case TokenType.STAR:
-      case TokenType.TILDE_SLASH:
-      case TokenType.PERCENT:
-      case TokenType.PLUS_EQ:
-      case TokenType.MINUS_EQ:
-      case TokenType.STAR_EQ:
-      case TokenType.TILDE_SLASH_EQ:
-      case TokenType.PERCENT_EQ:
-        if (t1 == typeProvider.intType && t2 == typeProvider.intType) return t1;
-        if (t1 == typeProvider.doubleType && t2 == typeProvider.doubleType)
-          return t1;
-        // This particular combo is not spelled out in the spec, but all
-        // implementations and analyzer seem to follow this.
-        if (t1 == typeProvider.doubleType && t2 == typeProvider.intType)
-          return t1;
-    }
-    return normalReturnType;
-  }
 }
 
 /// Checks for overriding declarations of fields and methods. This is used to