There seems to be an issue with how SystemJS detects module formats automatically.
It uses a regular expression to determine whether the source is in ES6 and requires transpilation:
// The regex used for detecting ES6 modules may not be accurate but covers most cases
var esmRegEx = /(^\s*|[}\);\n]\s*)(import\s*(['"]|(\*\s+as\s+)?[^"'\(\)\n;]+\s*from\s*['"]|\{)|export\s+\*\s+from\s+["']|export\s*(\{|default|function|class|var|const|let|async\s+function))/;
Interestingly, TypeScript versions 2.3.1 and 2.3.2 include a comment in their source code that matches this regex:
// In declaration files, exported modules might need to be accessed elsewhere as well. For example:
// declare module "a" { export type T = number; }
// declare module "b" { import { T } from "a"; export const x: T; }
So when troubleshooting this issue, you might notice that SystemJS loads a transpiler (TypeScript), recognizes it as needing transpilation for ES6, tries to load another transpiler, and ultimately skips transpiling your own code (main.ts) altogether.
To resolve this, specify the proper format for TypeScript as 'global' in the SystemJS configuration at the top level:
meta: {typescript: {format: 'global'}}