Merge pull request microsoft#3877 from Microsoft/port-3797-to-master

Port PR 3797 to master

Author: vladima

src/services/services.ts

@@ -91,6 +91,9 @@ namespace ts {
          * not happen and the entire document will be re - parsed.
          */
         getChangeRange(oldSnapshot: IScriptSnapshot): TextChangeRange;
+
+        /** Releases all resources held by this script snapshot */
+        dispose?(): void;
     }
 
     export module ScriptSnapshot {
@@ -1863,6 +1866,16 @@ namespace ts {
                     // after incremental parsing nameTable might not be up-to-date
                     // drop it so it can be lazily recreated later
                     newSourceFile.nameTable = undefined;
+
+                    // dispose all resources held by old script snapshot
+                    if (sourceFile !== newSourceFile && sourceFile.scriptSnapshot) {
+                        if (sourceFile.scriptSnapshot.dispose) {
+                            sourceFile.scriptSnapshot.dispose();
+                        }
+
+                        sourceFile.scriptSnapshot = undefined;
+                    }
+
                     return newSourceFile;
                 }
             }

src/services/shims.ts

@@ -34,6 +34,9 @@ namespace ts {
          * Or undefined value if there was no change.
          */
         getChangeRange(oldSnapshot: ScriptSnapshotShim): string;
+
+        /** Releases all resources held by this script snapshot */
+        dispose?(): void;
     }
 
     export interface Logger {
@@ -243,6 +246,14 @@ namespace ts {
             return createTextChangeRange(
                 createTextSpan(decoded.span.start, decoded.span.length), decoded.newLength);
         }
+
+        public dispose(): void {
+            // if scriptSnapshotShim is a COM object then property check becomes method call with no arguments
+            // 'in' does not have this effect
+            if ("dispose" in this.scriptSnapshotShim) {
+                this.scriptSnapshotShim.dispose();
+            }
+        }
     }
 
     export class LanguageServiceShimHostAdapter implements LanguageServiceHost {