Compile error: "No source code is available for type"

I am attempting to get the wave-in-a-box project building on Windows,
and am seeing errors in the GWT compilation.

Compiling module org.waveprotocol.box.webclient.WebClient
Validating newly compiled units
[ERROR] Errors in 'file:/D:/dev/wave/WPL-wave-protocol/proto_src/
org/waveprotocol/box/common/comms/WaveClientRpc.java'
[ERROR] Line 9: No source code is available for type
com.google.protobuf.ExtensionRegistry; did you forget to inherit a
required module?
[ERROR] Line 12: No source code is available for type
com.google.protobuf.GeneratedMessage; did you forget to inherit a
required module?
[ERROR] Line 33: No source code is available for type
com.google.protobuf.GeneratedMessage.FieldAccessorTable; did you
forget to inherit a required module?
[ERROR] Line 52: No source code is available for type
com.google.protobuf.CodedOutputStream; did you forget to inherit a
required module?

WaveClientRpc.java doesn't include any import statements at all, which
is clearly the problem, but I am not familiar enough with GWT to know
how to fix it.

I attempted to add
<inherits name="com.google.protobuf"/>
to the .gwt.xml file, but then the compiler complains that it can't
find protobuf.gwt.xml.

Any assistance would be greatly appreciated.

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