Fail on function definitions in contracts other than the main contract

test/aeso_compiler_tests.erl

@@ -573,6 +573,10 @@ failing_contracts() ->
          <<?Pos(2, 1)
            "Cannot compile with this version of the compiler,\n"
            "because it does not satisfy the constraint ", Version/binary, " == 9.9.9">>])
+    , ?TYPE_ERROR(multiple_contracts,
+         [<<?Pos(2, 3)
+            "Only the main contract can contain defined functions or entrypoints.\n"
+            "Fix: replace the definition of 'foo' by a type signature.">>])
     ].
 
 -define(Path(File), "code_errors/" ??File).

test/contracts/multiple_contracts.aes

@@ -0,0 +1,5 @@
+contract ContractOne =
+  entrypoint foo() = "foo"
+
+contract ContractTwo =
+  entrypoint bar() = "bar"