diff --git a/src/vs/workbench/api/common/extHostTreeViews.ts b/src/vs/workbench/api/common/extHostTreeViews.ts
index b9cf54fb746116a94096f81e8bd6fbfefeb6dc38..8621743e70652d1cd875314a56dc6dc1f1870d66 100644
--- a/src/vs/workbench/api/common/extHostTreeViews.ts
+++ b/src/vs/workbench/api/common/extHostTreeViews.ts
@@ -13,7 +13,7 @@ import { ExtHostTreeViewsShape, MainThreadTreeViewsShape } from './extHost.proto
 import { ITreeItem, TreeViewItemHandleArg, ITreeItemLabel, IRevealOptions } from 'vs/workbench/common/views';
 import { ExtHostCommands, CommandsConverter } from 'vs/workbench/api/common/extHostCommands';
 import { asPromise } from 'vs/base/common/async';
-import { TreeItemCollapsibleState, ThemeIcon } from 'vs/workbench/api/common/extHostTypes';
+import { TreeItemCollapsibleState, ThemeIcon, MarkdownString as MarkdownStringType } from 'vs/workbench/api/common/extHostTypes';
 import { isUndefinedOrNull, isString } from 'vs/base/common/types';
 import { equals, coalesce } from 'vs/base/common/arrays';
 import { ILogService } from 'vs/platform/log/common/log';
@@ -24,10 +24,6 @@ import { IMarkdownString } from 'vs/base/common/htmlContent';
 
 type TreeItemHandle = string;
 
-function isMarkdownString(value: any): value is vscode.MarkdownString {
-	return (value !== undefined) && value.appendCodeblock && value.appendMarkdown && value.appendText && (value.value !== undefined);
-}
-
 function toTreeItemLabel(label: any, extension: IExtensionDescription): ITreeItemLabel | undefined {
 	if (isString(label)) {
 		return { label };
@@ -531,7 +527,7 @@ class ExtHostTreeView<T> extends Disposable {
 	}
 
 	private getTooltip(tooltip?: string | vscode.MarkdownString): string | IMarkdownString | undefined {
-		if (isMarkdownString(tooltip)) {
+		if (MarkdownStringType.isMarkdownString(tooltip)) {
 			checkProposedApiEnabled(this.extension);
 			return MarkdownString.from(tooltip);
 		}
diff --git a/src/vs/workbench/api/common/extHostTypes.ts b/src/vs/workbench/api/common/extHostTypes.ts
index 80e6800171e5251d56b979356e338f4d8a1b696e..e6a5897675f4ed99e9952da9a7ed61b81837b0a4 100644
--- a/src/vs/workbench/api/common/extHostTypes.ts
+++ b/src/vs/workbench/api/common/extHostTypes.ts
@@ -1271,6 +1271,13 @@ export class MarkdownString {
 		this.value += '\n```\n';
 		return this;
 	}
+
+	static isMarkdownString(thing: any): thing is vscode.MarkdownString {
+		if (thing instanceof MarkdownString) {
+			return true;
+		}
+		return (thing !== undefined) && thing.appendCodeblock && thing.appendMarkdown && thing.appendText && (thing.value !== undefined);
+	}
 }
 
 @es5ClassCompat