feat: add dynamicImportFunction option

This option allows users of `es` format to rename the `import()`
function, which right now has mixed support in browsers. By allowing
this function to be renamed we can have polyfills (which cannot use the
reserved `import` keyword but could go by a different name like
`importModule`).

src/Chunk.ts

@@ -659,7 +659,8 @@ export default class Chunk {
 			freeze: options.freeze !== false,
 			indent: this.indentString,
 			namespaceToStringTag: options.namespaceToStringTag === true,
-			varOrConst: options.preferConst ? 'const' : 'var'
+			varOrConst: options.preferConst ? 'const' : 'var',
+			dynamicImportFunction: options.dynamicImportFunction || 'import'
 		};
 
 		// Make sure the direct dependencies of a chunk are present to maintain execution order

src/ast/nodes/Import.ts

@@ -11,10 +11,10 @@ interface DynamicImportMechanism {
 	interopRight?: string;
 }
 
-const getDynamicImportMechanism = (format: string, compact: boolean): DynamicImportMechanism => {
-	switch (format) {
+const getDynamicImportMechanism = (options: RenderOptions): DynamicImportMechanism => {
+	switch (options.format) {
 		case 'cjs': {
-			const _ = compact ? '' : ' ';
+			const _ = options.compact ? '' : ' ';
 			return {
 				left: 'Promise.resolve(require(',
 				right: '))',
@@ -23,9 +23,9 @@ const getDynamicImportMechanism = (format: string, compact: boolean): DynamicImp
 			};
 		}
 		case 'amd': {
-			const _ = compact ? '' : ' ';
-			const resolve = compact ? 'c' : 'resolve';
-			const reject = compact ? 'e' : 'reject';
+			const _ = options.compact ? '' : ' ';
+			const resolve = options.compact ? 'c' : 'resolve';
+			const reject = options.compact ? 'e' : 'reject';
 			return {
 				left: `new Promise(function${_}(${resolve},${_}${reject})${_}{${_}require([`,
 				right: `],${_}${resolve},${_}${reject})${_}})`,
@@ -38,6 +38,11 @@ const getDynamicImportMechanism = (format: string, compact: boolean): DynamicImp
 				left: 'module.import(',
 				right: ')'
 			};
+		case 'es':
+			return {
+				left: `${options.dynamicImportFunction}(`,
+				right: ')'
+			};
 	}
 };
 
@@ -78,7 +83,7 @@ export default class Import extends NodeBase {
 			return;
 		}
 
-		const importMechanism = getDynamicImportMechanism(options.format, options.compact);
+		const importMechanism = getDynamicImportMechanism(options);
 		if (importMechanism) {
 			const leftMechanism =
 				(this.resolutionInterop && importMechanism.interopLeft) || importMechanism.left;

src/rollup/types.d.ts

@@ -321,6 +321,7 @@ export interface OutputOptions {
 	sourcemapFile?: string;
 	sourcemapPathTransform?: (sourcePath: string) => string;
 	strict?: boolean;
+	dynamicImportFunction?: string;
 }
 
 export type WarningHandler = (warning: string | RollupWarning) => void;

src/utils/mergeOptions.ts

@@ -260,6 +260,7 @@ function getOutputOptions(
 		sourcemapExcludeSources: getOption('sourcemapExcludeSources'),
 		sourcemapFile: getOption('sourcemapFile'),
 		sourcemapPathTransform: getOption('sourcemapPathTransform'),
-		strict: getOption('strict', true)
+		strict: getOption('strict', true),
+		dynamicImportFunction: getOption('dynamicImportFunction', 'import')
 	};
 }

src/utils/renderHelpers.ts

@@ -9,6 +9,7 @@ export interface RenderOptions {
 	indent: string;
 	namespaceToStringTag: boolean;
 	varOrConst: 'var' | 'const';
+	dynamicImportFunction: string;
 }
 
 export interface NodeRenderOptions {