Fix TS 4.3 compile errors

For https://github.com/microsoft/TypeScript/issues/43419

- Works around type assertions not catching a method can modify the type
- Add more explicit typings

src/vs/base/test/browser/ui/tree/indexTreeModel.test.ts

@@ -325,12 +325,18 @@ suite('IndexTreeModel', () => {
 		assert.strictEqual(list[1].collapsible, false);
 
 		model.splice([0, 0], 1, [], options);
-		assert.strictEqual(list[0].collapsible, false);
-		assert.strictEqual(list[1], undefined);
+		{
+			const [first, second] = list;
+			assert.strictEqual(first.collapsible, false);
+			assert.strictEqual(second, undefined);
+		}
 
 		model.splice([0, 0], 0, [{ element: 1 }], options);
-		assert.strictEqual(list[0].collapsible, true);
-		assert.strictEqual(list[1].collapsible, false);
+		{
+			const [first, second] = list;
+			assert.strictEqual(first.collapsible, true);
+			assert.strictEqual(second.collapsible, false);
+		}
 	}));
 
 	test('expand', () => withSmartSplice(options => {

src/vs/editor/test/common/model/tokensStore.test.ts

@@ -8,13 +8,13 @@ import { MultilineTokens2, SparseEncodedTokens, TokensStore2 } from 'vs/editor/c
 import { Range } from 'vs/editor/common/core/range';
 import { TextModel } from 'vs/editor/common/model/textModel';
 import { IIdentifiedSingleEditOperation } from 'vs/editor/common/model';
-import { MetadataConsts, TokenMetadata, FontStyle } from 'vs/editor/common/modes';
+import { MetadataConsts, TokenMetadata, FontStyle, ColorId } from 'vs/editor/common/modes';
 import { createTextModel } from 'vs/editor/test/common/editorTestUtils';
 import { LineTokens } from 'vs/editor/common/core/lineTokens';
 
 suite('TokensStore', () => {
 
-	const SEMANTIC_COLOR = 5;
+	const SEMANTIC_COLOR: ColorId = 5;
 
 	function parseTokensState(state: string[]): { text: string; tokens: MultilineTokens2; } {
 		let text: string[] = [];

src/vs/platform/markers/test/common/markerService.test.ts

@@ -189,7 +189,7 @@ suite('Marker Service', () => {
 			endLineNumber: 1,
 			endColumn: 5,
 			message: 'test',
-			severity: 0,
+			severity: 0 as MarkerSeverity,
 			source: 'me'
 		};
 		let service = new markerService.MarkerService();

src/vs/workbench/contrib/externalTerminal/node/externalTerminalService.ts

@@ -341,7 +341,7 @@ export class LinuxExternalTerminalService implements IExternalTerminalService {
 /**
  * tries to turn OS errors into more meaningful error messages
  */
-function improveError(err: Error): Error {
+function improveError(err: Error & { errno?: string, path?: string }): Error {
 	if ('errno' in err && err['errno'] === 'ENOENT' && 'path' in err && typeof err['path'] === 'string') {
 		return new Error(nls.localize('ext.term.app.not.found', "can't find terminal application '{0}'", err['path']));
 	}