Add support for flow typecast comments

src/language-js/printer-estree.js

@@ -155,7 +155,7 @@ function genericPrint(path, options, printPath, args) {
     const node = path.getValue();
     if (hasFlowShorthandAnnotationComment(node)) {
       parts.push(" /*");
-      parts.push(node.trailingComments[0].value);
+      parts.push(node.trailingComments[0].value.trimLeft());
       parts.push("*/");
       node.trailingComments[0].printed = true;
       parts.push(")");

src/language-js/utils.js

@@ -8,7 +8,15 @@ function hasFlowShorthandAnnotationComment(node) {
     node.extra &&
     node.extra.parenthesized &&
     node.trailingComments &&
-    node.trailingComments[0].value.trimLeft().startsWith(":")
+    // We intentionally only match spaces and tabs here instead of any whitespace because
+    // Flow annotation comments cannot be split across lines. For example:
+    //
+    // (this /*
+    // : any */).foo = 5;
+    //
+    // is not picked up by Flow, so removing the newline would create a type annotation
+    // that the user did not intend to create.
+    node.trailingComments[0].value.match(/^[ \t]*:/)
   );
 }
 

tests/flow_comments/__snapshots__/jsfmt.spec.js.snap

@@ -91,12 +91,26 @@ new (window.Request/*: Class<Request> */)("");
 
 (this/*: any */).foo = 5;
 
+(this/*    : any */).foo = 5;
+
+// This next example illustrates that Prettier will not remove a line break
+// and unintentionally create a type annotation that was not there before.
+(this/*
+: any */).foo = 5;
+
 const x = (input /*: string */) /*: string */ => {};
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 new (window.Request /*: Class<Request> */)("");
 
 (this /*: any */).foo = 5;
 
+(this /*: any */).foo = 5;
+
+// This next example illustrates that Prettier will not remove a line break
+// and unintentionally create a type annotation that was not there before.
+this /*
+: any */.foo = 5;
+
 const x = (input /*: string */) /*: string */ => {};
 
 `;

tests/flow_comments/type_annotations.js

@@ -2,4 +2,11 @@ new (window.Request/*: Class<Request> */)("");
 
 (this/*: any */).foo = 5;
 
+(this/*    : any */).foo = 5;
+
+// This next example illustrates that Prettier will not remove a line break
+// and unintentionally create a type annotation that was not there before.
+(this/*
+: any */).foo = 5;
+
 const x = (input /*: string */) /*: string */ => {};