fix #26912, correct handling of op assign

pkg/analyzer/lib/src/generated/static_type_analyzer.dart

Index: pkg/analyzer/lib/src/generated/static_type_analyzer.dart
diff --git a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
index 103b2cbc1d82c5dbdf012c634ded0aba5ad5cb69..19019da8d839d5cceb3377b7a987e20d113c4021 100644
--- a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
+++ b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
@@ -258,13 +258,23 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     } else {
       ExecutableElement staticMethodElement = node.staticElement;
       DartType staticType = _computeStaticReturnType(staticMethodElement);
-      staticType =
-          _refineAssignmentExpressionType(node, staticType, _getStaticType);
+      staticType = _typeSystem.refineBinaryExpressionType(
+          _typeProvider,
+          node.leftHandSide.staticType,
+          operator,
+          node.rightHandSide.staticType,
+          staticType);
       _recordStaticType(node, staticType);
       MethodElement propagatedMethodElement = node.propagatedElement;
       if (!identical(propagatedMethodElement, staticMethodElement)) {
         DartType propagatedType =
             _computeStaticReturnType(propagatedMethodElement);
+        propagatedType = _typeSystem.refineBinaryExpressionType(
+            _typeProvider,
+            node.leftHandSide.propagatedType,
+            operator,
+            node.rightHandSide.propagatedType,
+            propagatedType);
         _resolver.recordPropagatedTypeIfBetter(node, propagatedType);
       }
     }
@@ -2232,44 +2242,6 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
   }
 
   /**
-   * Attempts to make a better guess for the type of the given assignment
-   * [expression], given that resolution has so far produced the [currentType].
-   * The [typeAccessor] is used to access the corresponding type of the left
-   * and right operands.
-   */
-  DartType _refineAssignmentExpressionType(AssignmentExpression expression,
-      DartType currentType, DartType typeAccessor(Expression node)) {
-    Expression leftHandSize = expression.leftHandSide;
-    Expression rightHandSide = expression.rightHandSide;
-    TokenType operator = expression.operator.type;
-    DartType intType = _typeProvider.intType;
-    if (typeAccessor(leftHandSize) == intType) {
-      // int op= double
-      if (operator == TokenType.MINUS_EQ ||
-          operator == TokenType.PERCENT_EQ ||
-          operator == TokenType.PLUS_EQ ||
-          operator == TokenType.STAR_EQ) {
-        DartType doubleType = _typeProvider.doubleType;
-        if (typeAccessor(rightHandSide) == doubleType) {
-          return doubleType;
-        }
-      }
-      // int op= int
-      if (operator == TokenType.MINUS_EQ ||
-          operator == TokenType.PERCENT_EQ ||
-          operator == TokenType.PLUS_EQ ||
-          operator == TokenType.STAR_EQ ||
-          operator == TokenType.TILDE_SLASH_EQ) {
-        if (typeAccessor(rightHandSide) == intType) {
-          return intType;
-        }
-      }
-    }
-    // default
-    return currentType;
-  }
-
-  /**
    * Create a table mapping HTML tag names to the names of the classes (in 'dart:html') that
    * implement those tags.
    *