Added fixConvertConstToLet.ts with preliminary fix, needs improved diagnostic messages

Author: Murad Khan

src/services/codefixes/fixConvertConstToLet.ts

@@ -0,0 +1,47 @@
+/* @internal */
+namespace ts.codefix {
+    const fixId = "fixConvertConstToLet";
+    const errorCodes = [
+        Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property.code
+    ];
+
+    registerCodeFix({
+        errorCodes,
+        getCodeActions(context) {
+            const { sourceFile, span } = context;
+            const { declaration, kind } = getDeclaration(sourceFile, span.start);
+            if (!declaration) return undefined;
+
+            const changes = textChanges.ChangeTracker.with(context, t => doChange(t, sourceFile, node, kind));
+            return [createCodeFixAction(fixId, changes, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property, fixId, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property)];
+        },
+        fixIds: [fixId],
+        getAllCodeActions: context => codeFixAll(context, errorCodes, (changes, diag) => {
+            const { kind, declaration } = getDeclaration(diag.file, diag.start, context);
+            if (declaration) doChange(changes, context.sourceFile, declaration, kind);
+        }),
+    });
+
+    function getDeclaration(sourceFile: SourceFile, pos: number, context: CodeFixContextBase): {declaration: Node, kind: SyntaxKind} {
+        const node = getTokenAtPosition(sourceFile, pos);
+        let declaration: Node;
+        let kind: SyntaxKind;
+        if (isVarConst(node)) {
+            declaration = findAncestor(node, isVariableDeclaration);
+            kind = SyntaxKind.ConstKeyword;
+        } else if (hasReadOnlyModifier(node)) {
+            declaration = findAncestor(node, isPropertyDeclaration);
+            kind = SyntaxKind.ReadonlyKeyword;
+        }
+        return { kind, declaration };
+    }
+
+    function doChange(changes: textChanges.ChangeTracker, sourceFile: SourceFile, node: Node, kind: SyntaxKind) {
+        if (kind === SyntaxKind.ConstKeyword) {
+            let letNode: Node = createNode(SyntaxKind.LetKeyword, node.getStart(), node.getEnd(), node.parent);
+            changes.replaceNode(sourceFile, node, letNode);
+        } else if (kind === SyntaxKind.ReadonlyKeyword) {
+            // let 
+        }
+    }
+}