fix(custom-element): fix event listeners with capital letter event names on custom elements

close https://github.com/vuejs/docs/issues/1708
close https://github.com/vuejs/docs/pull/1890

packages/compiler-core/__tests__/transforms/transformElement.spec.ts

@@ -314,6 +314,37 @@ describe('compiler: element transform', () => {
     )
     expect(root.helpers).toContain(MERGE_PROPS)
 
+    expect(node.props).toMatchObject({
+      type: NodeTypes.JS_CALL_EXPRESSION,
+      callee: MERGE_PROPS,
+      arguments: [
+        createObjectMatcher({
+          id: 'foo'
+        }),
+        {
+          type: NodeTypes.JS_CALL_EXPRESSION,
+          callee: TO_HANDLERS,
+          arguments: [
+            {
+              type: NodeTypes.SIMPLE_EXPRESSION,
+              content: `obj`
+            },
+            `true`
+          ]
+        },
+        createObjectMatcher({
+          class: 'bar'
+        })
+      ]
+    })
+  })
+
+  test('v-on="obj" on component', () => {
+    const { root, node } = parseWithElementTransform(
+      `<Foo id="foo" v-on="obj" class="bar" />`
+    )
+    expect(root.helpers).toContain(MERGE_PROPS)
+
     expect(node.props).toMatchObject({
       type: NodeTypes.JS_CALL_EXPRESSION,
       callee: MERGE_PROPS,
@@ -358,7 +389,8 @@ describe('compiler: element transform', () => {
             {
               type: NodeTypes.SIMPLE_EXPRESSION,
               content: `handlers`
-            }
+            },
+            `true`
           ]
         },
         {

packages/compiler-core/src/transforms/transformElement.ts

@@ -647,7 +647,7 @@ export function buildProps(
               type: NodeTypes.JS_CALL_EXPRESSION,
               loc,
               callee: context.helper(TO_HANDLERS),
-              arguments: [exp]
+              arguments: isComponent ? [exp] : [exp, `true`]
             })
           }
         } else {

packages/compiler-core/src/transforms/vOn.ts

@@ -47,12 +47,17 @@ export const transformOn: DirectiveTransform = (
       if (rawName.startsWith('vue:')) {
         rawName = `vnode-${rawName.slice(4)}`
       }
-      // for all event listeners, auto convert it to camelCase. See issue #2249
-      eventName = createSimpleExpression(
-        toHandlerKey(camelize(rawName)),
-        true,
-        arg.loc
-      )
+      const eventString =
+        node.tagType === ElementTypes.COMPONENT ||
+        rawName.startsWith('vnode') ||
+        !/[A-Z]/.test(rawName)
+          ? // for component and vnode lifecycle event listeners, auto convert
+            // it to camelCase. See issue #2249
+            toHandlerKey(camelize(rawName))
+            // preserve case for plain element listeners that have uppercase
+            // letters, as these may be custom elements' custom events
+          : `on:${rawName}`
+      eventName = createSimpleExpression(eventString, true, arg.loc)
     } else {
       // #2388
       eventName = createCompoundExpression([