Implement support for declare on class fields with Flow (#11178)

* Add parser support for Flow declare fields

* Add generator test

* Add "allowDeclareFields" option to flow-strip-types

* Add test

* Update error messages

* More tests

packages/babel-parser/src/plugins/flow.js

@@ -51,6 +51,10 @@ const FlowErrors = Object.freeze({
   AmbiguousDeclareModuleKind:
     "Found both `declare module.exports` and `declare export` in the same module. Modules can only have 1 since they are either an ES module or they are a CommonJS module",
   AssignReservedType: "Cannot overwrite reserved type %0",
+  DeclareClassElement:
+    "The `declare` modifier can only appear on class fields.",
+  DeclareClassFieldInitializer:
+    "Initializers are not allowed in fields with the `declare` modifier.",
   DuplicateDeclareModuleExports: "Duplicate `declare module.exports` statement",
   EnumBooleanMemberNotInitialized:
     "Boolean enum members need to be initialized. Use either `%0 = true,` or `%0 = false,` in enum `%1`.",
@@ -2085,6 +2089,39 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       }
     }
 
+    parseClassMember(
+      classBody: N.ClassBody,
+      member: any,
+      state: { hadConstructor: boolean },
+      constructorAllowsSuper: boolean,
+    ): void {
+      const pos = this.state.start;
+      if (this.isContextual("declare")) {
+        if (this.parseClassMemberFromModifier(classBody, member)) {
+          // 'declare' is a class element name
+          return;
+        }
+
+        member.declare = true;
+      }
+
+      super.parseClassMember(classBody, member, state, constructorAllowsSuper);
+
+      if (member.declare) {
+        if (
+          member.type !== "ClassProperty" &&
+          member.type !== "ClassPrivateProperty"
+        ) {
+          this.raise(pos, FlowErrors.DeclareClassElement);
+        } else if (member.value) {
+          this.raise(
+            member.value.start,
+            FlowErrors.DeclareClassFieldInitializer,
+          );
+        }
+      }
+    }
+
     // ensure that inside flow types, we bypass the jsx parser plugin
     getTokenFromCode(code: number): void {
       const next = this.input.charCodeAt(this.state.pos + 1);