Shrink span for convert-to-es6-module suggestion (microsoft#23441)

Author: Andy

src/services/suggestionDiagnostics.ts

@@ -6,7 +6,7 @@ namespace ts {
         const diags: Diagnostic[] = [];
 
         if (sourceFile.commonJsModuleIndicator) {
-            diags.push(createDiagnosticForNode(sourceFile.commonJsModuleIndicator, Diagnostics.File_is_a_CommonJS_module_it_may_be_converted_to_an_ES6_module));
+            diags.push(createDiagnosticForNode(getErrorNodeFromCommonJsIndicator(sourceFile.commonJsModuleIndicator), Diagnostics.File_is_a_CommonJS_module_it_may_be_converted_to_an_ES6_module));
         }
 
         const isJsFile = isSourceFileJavaScript(sourceFile);
@@ -61,4 +61,8 @@ namespace ts {
                 return undefined;
         }
     }
+
+    function getErrorNodeFromCommonJsIndicator(commonJsModuleIndicator: Node): Node {
+        return isBinaryExpression(commonJsModuleIndicator) ? commonJsModuleIndicator.left : commonJsModuleIndicator;
+    }
 }

tests/cases/fourslash/refactorConvertToEs6Module_export_named.ts

@@ -3,7 +3,7 @@
 // @allowJs: true
 
 // @Filename: /a.js
-////[|exports.f = function() {}|];
+////[|exports.f|] = function() {};
 ////exports.C = class {};
 ////exports.x = 0;
 ////exports.a1 = () => {};