With the fix for dart-lang/sdk#56820 / #5157, the breakpoint will no longer be set. The VM returns us a reason:

{
	"jsonrpc": "2.0",
	"error": {
		"code": 102,
		"message": "Cannot add breakpoint",
		"data": {
			"request": {
				"method": "addBreakpointWithScriptUri",
				"params": {
					"isolateId": "isolates/49774056015291",
					"scriptUri": "file:///D:/Dev/Test%20Projects/dartcode_5157/bin/dartcode_5157.dart",
					"line": "8"
				}
			},
			"details": "addBreakpointWithScriptUri: Cannot add breakpoint at line 8. Error occurred when resolving breakpoint location: No debuggable code where breakpoint was requested."
		}
	},
	"id": "15"
}

We should wire this up to the reason field on the breakpoint so that it shows if the user hovers over the grey breakpoint.